class Task50:
    def run(self, string):
        # assert every character is in 'a'-'z'
        buffer = [0 for _ in range(26)]
        for char in string:
            buffer[ord(char) - ord('a')] += 1
        for char in string:
            if buffer[ord(char) - ord('a')] == 1:
                return char
        return None
